The present invention relates to a transmission unit composed so as to transmit the engine output to a speed change mechanism via a torque converter and having a friction engagement element inside the speed change mechanism, which is engaged and actuated by receiving the hydraulic force when the vehicle is started and sets the vehicle start stage. More specifically, the present invention relates to a vehicle start control apparatus for restarting the engine, which can be stopped under the prescribed conditions, from the state in which the engine was stopped and engaging the friction engagement element and setting the vehicle start stage by supplying hydraulic oil to the friction engagement element.
Vehicles in which engine fuel economy can be improved and the amount of exhaust gases and noise can be reduced by conducting control so as to stop the engine when the vehicle is stopped, accelerator throttle is completely closed, and brakes are applied, have been developed and used in practice. Such vehicles are typically controlled so that if the accelerator pedal is stamped on from the state in which the engine was stopped, the engine is immediately restarted and the vehicle is started.
In such vehicles, automatic transmissions have been used as transmission units. In automatic transmissions, a speed change stage is set by a friction engagement element (hydraulic clutch, hydraulic brake, and the like) which receives the oil pressure and is engaged and actuated thereby. However, the friction engagement element is engaged and actuated by receiving the oil pressure from a hydraulic pump impeller driven by an engine, and when the engine is stopped, the oil pressure supplied to the friction engagement element is at a zero level. For this reason, when the engine is restarted and the vehicle is started, the oil pressure from the hydraulic pump impeller rotated by the engine is rapidly supplied to the friction engagement element for setting the vehicle start stage and a control providing for a smooth start of the vehicle is required.
With the foregoing in view, Japanese Patent Application Laid-open No. 11-351372 disclosed a method for a rapid pressure increase control conducted to supply the oil pressure for engaging the clutch for setting the vehicle start stage when the engine is restarted, thereby providing for a vehicle start control without any time delay, and also disclosed a method for changing the implementation mode of the rapid pressure increases control according to the type of the speed change stage serving as a vehicle start stage. Furthermore, Japanese Patent Application Laid-open No. 2000-35122 disclosed a method for a learning control by which when the rapid pressure increase control is thus conducted, a decision is made as to whether the implementation mode thereof is appropriate and the decision content is reflected on the next rapid pressure increase control. The above-mentioned rapid pressure increase control is designed to supply rapidly the hydraulic oil into the oil channel or hydraulic chamber so that the engagement of the friction engagement element can be initiated from the state in which the oil pressure thereof is at a zero level and to transfer the friction engagement element rapidly into a state just before the engagement.
However, the problem was that after the friction engagement element was thus transferred into a state just before the engagement, it was rapidly engaged. As a result, an engagement shock was generated and a smooth vehicle start control could not be conducted. Therefore, it is necessary to select appropriately the increase ratio of the oil pressure supplied to the friction engagement element. In order to suppress the engagement shock, the oil pressure increase ratio has to be decreased. However, if the oil pressure increase ratio is decreased, the engagement time is extended, causing the vehicle start delay. In order to resolve the problem associated with those mutually exclusive requirements, a control method can be considered by which a comparatively high oil pressure increase ratio is set at the initial stage of engagement and the engagement of the initial stage, which is not related to the engagement shock, is conducted at a high speed, but the oil pressure increase ratio is decreased from the stage at which the engagement force greatly increases.
When the engine is restarted and the vehicle start control is conducted, the oil pressure is not supplied to the friction engagement element setting the vehicle start stage immediately after the engine was restarted. Therefore, the input/output rotation speed difference (slip rotation) of the friction engagement element increases according to the increase in the engine rotation speed. If then the engagement of the friction engagement element is initiated, the input/output rotation speed difference gradually switches from the increase to the decrease and eventually becomes zero (full engagement). If a control is conducted so that the oil pressure is initially supplied at a high oil pressure increase ratio and then the increase ratio of the supplied oil pressure is decreased from the instant the engagement force of the friction engagement element increases and the input/output rotation speed difference switches from the increase to the decrease, the shock-free smooth engagement can be conducted without any time delay. However, the problem is that when the input/output rotation speed difference switches from the increase to the decrease, the variation ratio of the rotation speed difference is small. As a result, the instant the rotation speed difference switches from the increase to the decrease is difficult to detect accurately from the rotation speed difference and the control initiation timing of changing (decreasing) the increase ratio of the supplied oil pressure shifts easily.
It is an object of the present invention to provide an engine restart and vehicle start control apparatus with a structure such that timing of changing the increase ratio of the supplied oil pressure to the friction engagement element for setting the vehicle start stage during engine restarting can be set accurately and the friction engagement element can be engaged without time delay and shocks.
In accordance with the present invention, a transmission unit comprises an engine that can be stop controlled under the prescribed conditions, a torque converter connected to the output shaft of the engine, and a speed change mechanism which is connected to the output shaft of the torque converter and changes the speed and transmits the output rotation thereof, this speed change mechanism comprising a friction engagement element which is engaged and actuated by receiving a hydraulic force when the vehicle is started and sets a vehicle start stage (for example, a LOW clutch). An engine restart and vehicle start control apparatus in this transmission unit comprises pump impeller suction torque calculating means for calculating the pump impeller suction torque of the torque converter and oil pressure supply control means (for example, a control valve CV having an electromagnetic valve for controlling the oil pressure supplied to the LOW clutch) that can set an increase ratio of the oil pressure supplied to the friction engagement element, wherein when the engine is restarted from an engine stopped state and the vehicle start stage is set by supplying a hydraulic fluid to the friction engagement element and engaging the friction engagement element, the oil pressure supply control means conducts oil pressure supply at a first oil pressure increase ratio to the friction engagement element till the calculated pump impeller suction torque reaches a prescribed criterial value, and conducts oil pressure supply at a second oil pressure increase ratio which is less than the first oil pressure increase ratio from the instant the calculated pump impeller suction torque becomes higher than the prescribed criterial value.
In the transmission unit of the above-described structure, the pump impeller suction torque of torque converter is in the action-counteraction relationship with the engagement force of the friction engagement element, and the pump impeller suction torque of torque converter increases with the increase of the clutch engagement force. As a result, when the engine is restarted from an engine stopped state and the vehicle start stage is set by supplying hydraulic fluid to the friction engagement element and engaging the friction engagement element, the instant the friction engagement element moves from the initial engagement state to a state in which the engagement force increases can be determined accurately based on the pump impeller suction torque. With this in view, in accordance with the above-described present invention, a rapid delay-free engagement is performed by conducting the oil pressure supply at a first oil pressure increase ratio (a comparatively high increase ratio is set as this value) to the friction engagement element till the calculated pump impeller suction torque reaches the prescribed criterial value, and the rapid increase in the friction engagement force is suppressed and smooth, shock-free engagement is preformed by conducting the oil pressure supply at a second oil pressure increase ratio which is less than the first oil pressure increase ratio from the instant the calculated pump impeller suction torque becomes higher than the prescribed criterial value (that is, when the engagement force of the friction engagement element greatly increases).
Further, it is preferred that an oil pressure detector for detecting that the oil pressure supplied to the friction engagement element has become a prescribed low pressure is provided and that when the engine is restarted from an engine stopped state and the vehicle start stage is set by supplying hydraulic fluid to the friction engagement element and engaging the friction engagement element, the oil pressure supply control means conducts a control conducting a rapid oil pressure supply to the friction engagement element, for example, by setting the target supply oil pressure to a high pressure till the oil pressure supplied to the friction engagement element detected by the oil pressure detector becomes the prescribed low pressure and conducts an oil pressure supply at the first oil pressure increase ratio to the friction engagement element from the instant the oil pressure detector detects that the oil pressure supplied to the friction engagement element has become the prescribed low pressure. As a result, the friction engagement element can be rapidly set into a state immediately prior to engagement (a state in which the friction engagement element is filled with hydraulic oil to the prescribed low pressure) and the vehicle start control can be conducted without time delay.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.